Image capture devices may have a lens driver controlling an actuator that moves a lens assembly for adjusting focus in the image capture device. On example is a voice-coil motor (VCM). A VCM with spring return is a small, low-cost solution for auto-focusing (AF), and one of the simplest to implement. Movements using VCMs are repeatable and gearless, with lens position fixed by balancing motor and spring forces. The spring returns the lens to the infinite-focus position, and no power is dissipated unless focusing is required. A VCM is mechanically robust, shock-resistant, and has low-cost mechanics. VCMs can be used as a short-distance actuator in electronic devices such as the vibrating motor for a mobile phone, the vertical displacement actuator of a magnetic head or an optical pickup, the auto-focusing actuator for a camera, etc.
FIG. 1 is an exploded perspective view of a conventional voice coil motor (VCM) used in an image capture device for auto focusing (AF). As shown, the VCM includes an upper cover 100, a lower cover 105, and an outer frame 110 having four inner wall surfaces provided with a magnet 115 each, an internally threaded lens barrel 120 for a lens 125 to screw into, a coil 130 wound around an outer side of the lens barrel 120, an upper spring 135 and a lower spring 135. The lens barrel 120 is supported by the upper and the lower springs 135 to movably locate within the outer frame 110 and be surrounded by the four magnets 115.
In operation, as coil 130 is supplied with a current to produce a magnetic field, the lens barrel 120 and the four magnets 115 mutually repulse or attract to thereby produce a push force against the lens barrel 120. Since the lens barrel 120 is supported by the springs 135, the lens barrel 120 is displaced forward or backward within the outer frame 110 by the push force. Thus, the lens 125 position is fixed by balancing motor and spring forces. The springs 135 return the lens 125 to the infinite-focus position.
However, during VCM operation, any movement of lens 125 will cause vibration in the springs 135, known as “ring”. This ring occurs as the lens barrel 120 is actuated by the VCM. This ring is present as each of the springs 135 has an intrinsic vibration frequency that causes them to vibrate upon any lens 125 movement during VCM operation.
FIG. 2 is a graph of lens displacement as a function of time to illustrate the effects of ring on lens displacement. The sine wave in FIG. 2 represents an actual measurement, as a function of units of lens displacement over time, taken by an oscilloscope from an actual image capture device. As can be seen in FIG. 2, the continued vibration or ring in the VCM (due to springs 135) after a movement of lens 125 causes the lens 125 to continue to slightly wobble or vibrate for some time after VCM operation terminates (see in FIG. 2 at about 1200 μs on the x-axis). This sine wave indicative of lens displacement, which lessens in amplitude over time, is representative of the ring in an image capture device without any damping.
This vibration or ring can last anywhere from about approximately 50 milliseconds (ms) to approximately 200 ms, and can adversely affect auto-focus (AF) speed and accuracy. Thus, the ring can adversely affect desired image capture settings and hence image quality, if there has not been enough waiting time to permit the lens movement and any generated vibration or ring due to the movement, to terminate or cease prior to capturing an image.